The global positioning system (GPS) presently in use utilizes two carrier frequencies, 1.227 GHz (the L2 band) and 1.575 GHz (the L1 band), to transmit spread-spectrum signals from space vehicles, i.e., GPS satellites, to GPS receivers. The spectral densities of the signals are very small, on the order of −160 dBW/Hz. Because the carrier frequencies reside in an increasingly crowded band and the strength of the signal is so small, the GPS system is highly susceptible to interfering signals, intentionally or unintentionally directed toward the GPS receiver. In order to mitigate the effect of a potential interfering signal, phased array antennas have been developed to track the GPS space vehicles and to place nulls in the positions of interfering jammer signals. These phased array antenna systems require additional circuitry and complex algorithms to phase the elements of the array correctly and to track the jammers and/or the satellites.
There is a need for a simpler system. The antenna system described herein includes a spherical lens, with receiving elements, such as patch antennas, located on the hemispherical or approximately hemispherical focusing surface of the lens. Thus, hemispherical coverage of visible GPS satellites can be obtained. Furthermore, the signal from one GPS satellite will focus onto a spot and the signal will be picked up by one or more of the elements of the antenna and then combined with signals from the other elements to provide essentially omni-directional coverage. Thus, there is no need for circuitry to track the GPS satellites as is done with phased array technology. Nulling a jamming signal is easily performed by switches which are preferably co-located at each element which routes the offending signal to a load instead of the GPS receiver. The algorithm which determines when an element needs to be turned off can be as simple as a power detector.
An additional technique for increasing the robustness of the GPS system against interference includes a constellation of UAVs flown at a substantial distance from the GPS receiver. These UAVs have their own GPS receivers to determine their precise locations. This information is then re-coded in GPS format and placed on a microwave carrier or used to generate a spread spectrum signal. A line-of-site link is established with the GPS receiver, thus forming an extra tier to the GPS system. The GPS receiving antenna described herein could then be reduced in size due to the availability of the additional GPS information. The UAV's behave like a local GPS system. The advantage of this approach is that a jamming signal would have to be located very near one of the UAV's and would have to follow it in order effectively jam the disclosed antenna. In addition, the jammer would need to know the frequency of operation that the UAV is using, which could be varied by spread spectrum techniques. This approach of retransmission of the GPS information, coupled with the multiple beam switched null antenna system, provides a very secure GPS system which a jammer will find very difficult to interfere with.
The invention may be used in a number of different applications, including military, to provide more reliable GPS position information particularly in a noisy or jammed RF environment.
The prior art includes:    (1) N. Padros, J. I. Ortigosa, J. Baker, M. F. Iskander, and B. Thomberg, “Comparative Study of high-performance GPS Receiving Antenna Designs, IEEE Trans. Antennas and Propag., Vol. 45, No. 4, April 1997, pp. 698-706.    (2) R. L. Fante and J. J. Vacarro, “Cancellation of Jammers and Jammer Multipathy in a GPS Receiver,” IEEE AES Systems Magazine, November 1998, pp. 25-28.    (3) J. M. Blas, J. De Pablos, F. Perez, and J. I. Alonso, “GPS Adaptive Array for Use in Satellite MobileCommunications,” Satellite Systems for Mobile Communications and Navigation Conference Publication, May 13-15, 1996, pp. 28-31.
The aforementioned publications explain how phased arrays and algorithms can be used for tracking and adaptive nulling. The present invention does not require phased arrays or adaptive nulling.    (4) R. M. Rudish, J. S. Levy, and P. J. McVeigh, “Multiple Beam Antenna System and Method,” U.S. Pat. No. 6,018,316, Jan. 25, 2000.    (5) A. L. Sreenivas, “Spherical Lens Having an Electronically Steerable Beam”, U.S. Pat. No. 5,821,908. Oct. 13, 1998.
These patents describe systems that use lenses for beam steering. The use of lenses to form multiple beams is well known. The present invention does not use a lens to form beams for steering but rather for instantaneous omni-directional coverage. As is disclosed herein, a null is “steered”, although not in the phased array sense, by turning off elements that are receiving jamming signals. This is an important point of differentiation between the lens disclosed herein and prior art lenses. The lens disclosed herein works particularly well for GPS signals since the direction from which the interfering signal(s) is (are) coming does not need to be known, but rather the direction of the interfering source(s) are nulled without the need to specifically track the jammer(s).    (6) Ayyagari, J. P. Harrang, and S. Ray, “Airborne Broadband Communications Network, U.S. Pat. No. 6,018,659, Jan. 25, 2000    (7) M. M. Aguado, “Retransmitted GPS Interferometric System, U.S. Pat. No. 5,570,097, Oct. 29, 1996.